linux-kernel - Re: linux-next: boot warning after merge of the vfs-brauner tree

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Message-ID: <ZszrJkFOpiy5rCma@bombadil.infradead.org>
Date: Mon, 26 Aug 2024 13:52:54 -0700
From: Luis Chamberlain <mcgrof@...nel.org>
To: Christophe Leroy <christophe.leroy@...roup.eu>,
	Mike Rapoport <rppt@...nel.org>, Song Liu <song@...nel.org>,
	Arnd Bergmann <arnd@...db.de>
Cc: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Christian Brauner <brauner@...nel.org>,
	Pankaj Raghav <p.raghav@...sung.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Next Mailing List <linux-next@...r.kernel.org>,
	djwong@...nel.org, ritesh.list@...il.com,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: linux-next: boot warning after merge of the vfs-brauner tree

On Mon, Aug 26, 2024 at 07:43:20PM +0200, Christophe Leroy wrote:
> 
> 
> Le 26/08/2024 à 17:48, Pankaj Raghav (Samsung) a écrit :
> > On Mon, Aug 26, 2024 at 05:59:31PM +1000, Stephen Rothwell wrote:
> > > Hi all,
> > > 
> > > After merging the vfs-brauner tree, today's linux-next boot test (powerpc
> > > pseries_le_defconfig) produced this warning:
> > 
> > iomap dio calls set_memory_ro() on the page that is used for sub block
> > zeroing.
> > 
> > But looking at powerpc code, they don't support set_memory_ro() for
> > memory region that belongs to the kernel(LINEAR_MAP_REGION_ID).
> > 
> > /*
> >   * On hash, the linear mapping is not in the Linux page table so
> >   * apply_to_existing_page_range() will have no effect. If in the future
> >   * the set_memory_* functions are used on the linear map this will need
> >   * to be updated.
> >   */
> > if (!radix_enabled()) {
> >          int region = get_region_id(addr);
> > 
> >          if (WARN_ON_ONCE(region != VMALLOC_REGION_ID && region != IO_REGION_ID))
> >                  return -EINVAL;
> > }
> > 
> > We call set_memory_ro() on the zero page as a extra security measure.
> > I don't know much about powerpc, but looking at the comment, is it just
> > adding the following to support it in powerpc:
> > 
> > diff --git a/arch/powerpc/mm/pageattr.c b/arch/powerpc/mm/pageattr.c
> > index ac22bf28086fa..e6e0b40ba6db4 100644
> > --- a/arch/powerpc/mm/pageattr.c
> > +++ b/arch/powerpc/mm/pageattr.c
> > @@ -94,7 +94,9 @@ int change_memory_attr(unsigned long addr, int numpages, long action)
> >          if (!radix_enabled()) {
> >                  int region = get_region_id(addr);
> > -               if (WARN_ON_ONCE(region != VMALLOC_REGION_ID && region != IO_REGION_ID))
> > +               if (WARN_ON_ONCE(region != VMALLOC_REGION_ID &&
> > +                                region != IO_REGION_ID &&
> > +                                region != LINEAR_MAP_REGION_ID))
> >                          return -EINVAL;
> >          }
> >   #endif
> 
> By doing this you will just hide the fact that it didn't work.
> 
> See commit 1f9ad21c3b38 ("powerpc/mm: Implement set_memory() routines") for
> details. The linear memory region is not mapped using page tables so
> set_memory_ro() will have no effect on it.
> 
> You can either use vmalloc'ed pages, or do a const static allocation at
> buildtime so that it will be allocated in the kernel static rodata area.
> 
> By the way, your code should check the value returned by set_memory_ro(),
> there is some work in progress to make it mandatory, see
> https://github.com/KSPP/linux/issues/7

Our users expect contiguous memory [0] and so we use alloc_pages() here,
so if we're architecture limitted by this I'd rather we just remove the
set_memory_ro() only for PPC, I don't see why other have to skip this.

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index c02b266bba52..aba5cde89e14 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -775,14 +775,22 @@ EXPORT_SYMBOL_GPL(iomap_dio_rw);
 
 static int __init iomap_dio_init(void)
 {
+	int ret;
+
 	zero_page = alloc_pages(GFP_KERNEL | __GFP_ZERO,
 				IOMAP_ZERO_PAGE_ORDER);
 
 	if (!zero_page)
 		return -ENOMEM;
 
-	set_memory_ro((unsigned long)page_address(zero_page),
-		      1U << IOMAP_ZERO_PAGE_ORDER);
-	return 0;
+	if (IS_ENABLED(CONFIG_PPC))
+		return 0;
+
+	ret = set_memory_ro((unsigned long)page_address(zero_page),
+			    1U << IOMAP_ZERO_PAGE_ORDER);
+	if (ret)
+		free_pages((unsigned long) zero_page, IOMAP_ZERO_PAGE_ORDER);
+
+	return ret;
 }
 fs_initcall(iomap_dio_init);

Thoughts?

[0] https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git/commit/?h=vfs.blocksize&id=d940b3b7b76b409b0550fdf2de6dc2183f01526f

  Luis